1. Field of the Invention
The present invention is generally concerned with a resource reservation protocol in an asynchronous transfer mode (ATM) network.
2. Description of the Prior Art
In an asynchronous network resources are time-shared between a plurality of user terminals. Data units called cells form a stream with no time reference in respect of their relative positions. The cells carry identifiers of routing channels connecting user terminals respectively. Unlike the synchronous time-division technique, no predetermined time slot is assigned to the cells of a routing channel. A user terminal chooses a bit rate for the cells to be transmitted without direct reference to the network, the instantaneous bit rate between the terminal and the network being always the same.
According to the telecommunication network operators, ATM networks will grow in two stages. These two stages are shown diagrammatically in FIG. 1. During a first stage, a network operator will provide a "meshed" network offering a leased line service to each user terminal. The final "switched" network that will interconnect all user terminals will come about in a second stage.
The meshed network will provide a fast response to business users who represent a high demand for high bit rate communications. The meshed network is made up of switch-over units 10, 11 and 12 which switch-over virtual routing channels in a virtual path 30, 31, 32, 33 to another virtual path according to the leased line in question. A plurality of user terminals share the same virtual path which connects a particular two switch-over units.
The second stage will result from the addition to this meshed network infrastructure of ATM switches 20, 21 and 22 that will switch virtual channels between incoming and outgoing virtual paths of the switches. In this second stage, the offer will no longer be limited to a leased line service offering the user only a high bit rate link between two predetermined terminals. Each user terminal will have the benefit of a fully switched service that can connect it to any other user terminal.
In the meshed network a plurality of user terminals share the same virtual path. The main advantage of the asynchronous transfer mode is that each user terminal transmits data cells at its own bit rate. To optimize the bandwidth of the virtual path, a respective cell transmission bit rate is reserved by each user terminal before it transmits data cells. A bit rate fast reservation protocol has been defined whereby the bit rate in a virtual path requested by a given user terminal is constant or variable, and a bit rate modification can be requested by the user terminal even during a call already set up.
More particularly the invention relates to a method of bit rate reservation at switching nodes of an asynchronous transfer mode (ATM) network through which passes a routing channel having a current bit rate stored at the nodes and situated between first and second terminals. In each of node is received a reservation cell that contains a requested bit rate. The reservation cell transmitted from the first terminal through the network is received by each node, then is transmitted by an outgoing multiplex channel having a maximum bit rate through the network to the second terminal. In each of the nodes along the routing path is received an acknowledgement cell which is transmitted by the second terminal to the first terminal through the network in response to the retransmitted reservation cell.
A bit rate reservation method of this kind is described in U.S. Pat. No. 5,081,620. In this method a reservation cell and an acknowledgement cell constitute first and second call request packets. A resource, i.e. a bit rate, in the outgoing multiplex channel constituting a virtual path at each switching node through which the routing channel passes is allocated to the first terminal in each passed switching node when the requested bit rate is less than an available bit rate equal to the difference between a maximum disposable bit rate and a total of bit rates of other logical routing channels using the outgoing multiplex channel. Thus if the requested bit rate is near but below the available bit rate, there remains in practise no available resource to be allocated to other user terminals, whereas if the requested bit rate is higher than the available bit rate the connection is refused, even though part of the available bit rate might have been temporarily useful to the calling user.
The article "A reservation principle with applications to the ATM traffic control" by Pierre BOYER and Didier TRANCHIER published in "Computer Networks and ISDN Systems", vol. 24, no. 4, May 1992, Amsterdam, pages 321-334, describes a fast reservation protocol with delayed transmission (FRP-DT). A protocol of this kind is also intended for sources with a bit rate that varies in negociated steps.
A first user terminal is initially connected to a second user terminal by transmitting an allocation request in respect of a given bit rate. If the bit rate reserved needs to be increased or decreased during the call between the two terminals, the first terminal transmits a bit rate increase or decrease request cell. This cell is received by a protocol control unit associated with a plurality of terminals and located at the input of the ATM network. In response to a bit rate increase or decrease request cell, the control unit transmits a limited number of successive bit rate increase or decrease requests provided that previous requests have not been satisfied. In the case of a bit rate increase request, if the bit rate increase requested cannot be satisfied entirely within the network, the bit rate increase request is entirely rejected. Otherwise, the first terminal is allocated a greater bandwidth according to the requested bit rate increase. In all cases the current bit rate already allocated to the terminal is not contested and a bit rate decrease request constitutes a purely indicative procedure that is never rejected.
Thus in the aforementioned patent and article, a bit rate increase request, i.e. a request for a bit rate higher than the current bit rate already allocated to a virtual routing channel between two terminals, is not satisfied completely or is satisfied entirely, without consideration as to whether some of the available resource at the switching nodes could have been allocated to the calling terminal whose request has not been met and some other part of the available resource could have been retained for other terminals requesting resource.
Furthermore, the second terminal does not transmit any explicit reserved bit rate acknowledgement, which can lead to errors in the allocation of bit rates to the routing channels.